1. Field of the Invention
The present invention relates to an image-taking lens apparatus, and more particularly to an image-taking lens apparatus (used as a main component of, for example, a digital still camera, digital video camera, or camera incorporated in or externally fitted to a personal computer, mobile computer, cellular phone, personal digital assistant, or the like) that optically takes in an image of a subject through an image-taking lens system and then outputs it as an electrical signal by means of an image sensor. The present invention relates, in particular, to an image-taking lens apparatus provided with a compact image-taking lens system, and to a slim camera provided with such an image-taking lens apparatus.
2. Description of Related Art
In recent years, as personal computers and the like become increasingly widespread, digital still cameras, digital video cameras, and the like (hereinafter referred to simply as “digital cameras”), which permit easy acquisition of image information into digital appliances, have been becoming more and more popular among individual users. Digital cameras are expected to become still more popular in future as image information inputting devices. In general, the image quality of such a digital camera is determined by the number of pixels provided in the solid-state image sensor, such as a CCD (charge-coupled device), incorporated therein. Nowadays, digital cameras designed for general consumers offer high resolutions over 500 million pixels, and are thus approaching silver-halide film cameras in image quality. Accordingly, in the image-taking lens systems used in digital cameras, high optical performance is sought to cope with the increasingly high resolutions of image sensors.
On the other hand, even in digital cameras designed for general consumers, zooming of an image, in particular optical zooming with little image degradation, is sought, and in addition, for higher portability, slimming-down is also sought. The biggest bottleneck in slimming down a camera is the thickness of the image-taking lens system from the surface closest to the object to the image surface. A well-known technique for reducing this thickness is the so-called collapsible construction in which the image-taking lens system is moved out of the camera body when photographing is performed and is collapsed into the camera body when the camera is carried around. However, with the collapsible construction, it is impossible to make the thickness of the image-taking lens apparatus smaller than the sum of the thickness of the lens system itself, the thickness of the image sensor, and the thickness of the optical filter components required by the image sensor. This makes it impossible to make digital cameras satisfactorily compact. Moreover, with the collapsible construction, the lens needs to be moved out when the camera is used, and this requires a complicated lens barrel construction. This results in high manufacturing difficulty, leading to great degradation in image quality and a great increase in costs. Furthermore, in a construction where the lens is moved out after the camera power is turned on, it takes time to complete the preparations for photographing. This makes the user more likely to miss good opportunities for photographing.
To solve the problems mentioned above, zoom lens systems that attempt to slim down a camera as a whole by bending the optical path by using a prism provided in the middle of the optical system are proposed in the following patent publications:                Publication 1: Japanese Patent Application Laid-Open No. 2000-131610        Publication 2: Japanese Patent Application Laid-Open No. 2003-43354        Publication 3: Japanese Patent Application Laid-Open No. 2003-107356        Publication 4: Japanese Patent Application Laid-Open No. 2003-98430        
In the zoom lens systems disclosed in Publication 1, a prism having a refractive index of 1.569 or less is used. That is, the prism has too low a refractive index, making the total length of the zoom lens systems unduly long. Moreover, here, the technique of bending the optical axis is not used to effectively achieve slimming-down. Thus, the obtained image-taking lens systems cannot be said to be satisfactorily compact.
The zoom lens systems disclosed in Publications 2 to 4 adopt zoom constructions of the type (the so-called negative-led type) in which the first lens unit has a negative optical power. Most of these zoom lens systems include a plastic prism of which both the entrance-side and exit-side surfaces are given curvatures, and thus many of them are supposed to be difficult to manufacture. The zoom lens systems disclosed in Publication 3 include a glass prism of which both the entrance-side and exit-side surfaces are given curvatures, and such a glass prism is more difficult to manufacture than a plastic equivalent. When consideration is given to the technology of glass molding and of plastic injection molding and to the after-molding machining of a prism, giving curvatures to a prism involves many problems to be solved in terms of the manufacture of the prism. For example, manufacturing errors arising in integral molding may result in degraded performance, and the manufacturing difficulty of integral molding itself may result in higher costs. Moreover, the prisms used have refractive indices of 1.883 or less. Thus, the obtained image-taking lens systems cannot be said to be satisfactorily compact.